XCR was almost adopted by the us military and lost the bid to a dumb error. So it's obviously a good gun if the us military looked at it that closely.
Not sure where you got that, but it was never even close to being in the running.
What makes the sl8 better ?
The barrel to start with.
The sl8's are high quality polygonal rifled barrels. The xcr barrels look to be hand made in Pakistan.
I've run a bore scope down both (I've owned both rifles) and there is no comparison, that's a good part of the accuracy difference.
The sl8 is very solid feeling, the xcr has a bunch of slop between the upper and the lower. And the folding stock has some wiggle. Not good for accuracy.
The xcr has a ridiculous barrel attachment system, that's where the rest of the accuracy goes.
I had several screws fall out of the xcr, rendering the rifle unusable until I could locate more hardware.
If I had to buy one or the other again (I no longer own either), I'd go with the sl8, hands down.
The reasoning there is that a nonrestricted is more likely to be used for hunting, and therefore accuracy trumps ergonomics.
If you want maximum ergonomics in a non restricted, get a tavor.
If you want to land in the middle somewhere, get a rebarreled acr reregistered as non restricted.